Light olefin materials, including ethylene and propylene, represent a large portion of the worldwide demand in the petrochemical industry. Specifically, propylene demand in the petrochemical industry has grown substantially, largely due to its use as a precursor in the production of polypropylene for packaging materials and other commercial products. Other downstream uses of propylene include the manufacture of acrylonitrile, acrylic acid, acrolein, propylene oxide and glycols, plasticizer oxo alcohols, cumene, isopropyl alcohol, and acetone.
Propylene is typically produced during the steam cracking or pyrolysis of hydrocarbon feedstocks such as natural gas, petroleum liquids, and carbonaceous materials (e.g., coal, recycled plastics, and organic materials), to produce ethylene. Additional sources of propylene are byproducts of fluid catalytic cracking (FCC) and reside fluid catalytic cracking (RFCC), normally targeting gasoline production. FCC is described, for example, in U.S. Pat. No. 4,288,688 and elsewhere. A mixed, olefinic C3/C4 hydrocarbon byproduct stream of FCC may be purified in propylene to polymer grade specifications by the separation of C4 hydrocarbons, propane, ethane, and other compounds.
More recently, the desire for propylene and other light olefins from alternative, non-petroleum based feeds has led to the use of oxygenates such as alcohols and, more particularly, methanol, ethanol, and higher alcohols or their derivatives. Methanol, in particular, is useful in a methanol-to-olefin (MTO) conversion process described, for example, in U.S. Pat. No. 5,914,433. The yield of light olefins from such a process may be improved using olefin cracking to convert some or all of the C4+ product of MTO in an olefin cracking reactor, as described in U.S. Pat. No. 7,268,265. Flexibility in the MTO product selection to integrate the MTO plant within the existing petrochemical infrastructure is a challenge while designing new plants. The MTO process produce significant amount of C4 olefins, C5 olefins, aromatics and heavier species in addition to ethylene and propylene which are the desirable products. The effective utilization of these by-products can result in increased production of light olefins such as propylene and ethylene and can significantly improve the economics of the process. Also, C4 olefins may be used to provide other high value products such as butadiene.
In a typical MTO process employing SAPO-34, Propylene to Ethylene (P/E) product ratio at best can be 1.5. With the integration of Olefin cracking process (OCP), the P/E product ratio can be raised to about 2.0. Further, in an MTO process employing SAPO-18 and an operating pressure of 50 psig, the (P/E) product ratio is around 2:1 and with OCP integration (MTO-OCP) it can be raised between 2.5:1 to 3.0:1. Accordingly, under the best circumstances, with SAPO-18 and higher pressure in a MTO-OCP unit, the product composition will be 75% propylene and 25% ethylene.
However, there is a great demand for increase in propylene product specifically. The competitive route of Lurgi MTP® (methanol-to-propylene) process produces about 65% propylene, 25% gasoline C5+ fraction and balance C4− light ends. It is easier to sell byproduct gasoline than ethylene. It can be sent by rail car or truck to customer locations. For SAPO based MTO process, ethylene is not easy to transport, or the propylene producer also must get into ethylene derivative business and install an ethylene conversion process. This requires significant additional capital.
Another alternative is to install a C2-C4 olefin metathesis process plant and convert it to propylene. The catalyst for the metathesis process is very sensitive to impurities and MTO C4 fraction will require extensive feed treatment. The process is also somewhat complex and requires significant more capital.
Accordingly, it is desirable to effectively utilize by-products in an MTO-OCP process for increased production of propylene and achieve higher P/E product ratio. Further, it is desirable to increase production of propylene in an oxygenate to olefin process with lower capital and operating cost requirements. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the subject matter.